1. Field of the Invention
The present invention relates to a method of employing RF energy and a cooperating antenna—non-linear device combination to alter an interrogating frequency in a specific manner so as to provide a specific indication if a particular article of interest is present. In another embodiment the invention provides such a method wherein a physical condition, such as temperature or pressure, will cause responsive alternation of the reflected RF energy and thereby permit a determination of such properties. The invention also relates to corresponding apparatus.
2. Description of the Prior Art
It has long been known to provide non-contacting means for identification or informational purposes. For example, bar codes and cooperating readers have been employed in connection with product identification, price determination, inventory control, identification of individuals or articles, and numerous other purposes.
It has been known to provide radio frequency identification bar code systems which require the use of complimentary metal oxides semiconductor (CMOS) chips for operation.
U.S. Pat. No. 3,798,642 discloses a recognition system for identifying one or more groups of harmonic generating targets which system can be used for personnel or object identification. The system involves transmitting an amplitude modulated signal to a target which responsively generates a harmonic signal having a plurality of signal components of respective amplitudes and means for varying the amplitude of at least one of the signal components with receiver means sensing whether a pre-selected coded arrangement of amplitudes was present.
U.S. Pat. No. 5,381,137 discloses an RF tagging system which has a plurality of resident circuits on a tag with the system determining a resonant frequency of each resonant circuit and producing a corresponding code. It is said to be useful for coding articles such as baggage or production inventory. The radiated signals are said to preferably be phase shifted during the detection process.
It has been known to provide a polymer composition having an electrically conductive filler such that application of forces thereto results in a change in electrical resistance of the material. See U.S. Pat. No. 6,291,568.
It has been known to provide systems wherein RF energy may be transmitted from a base station to a remote station with retransmission from said remote station to said base station being provided to present information to the base station. See U.S. Pat. No. 6,289,237.
In spite of the foregoing known systems, there remains a very real and substantial need for a simple, reliable person or article identification method and related apparatus which avoids the need for use of remote CMOS chips and bar codes. There also remains a need for monitoring methods and apparatus for use in remotely monitoring physical conditions such as pressure or temperature, for example.